1. Field of the Invention
This invention relates to sound recording for motion pictures, and more particularly to a method and apparatus for detecting crossmodulation distortion on optical sound tracks.
2. Description of the Prior Art
Variable area motion picture sound tracks exhibit a form of distortion peculiar to optical sound tracks if the negative and print are not exposed and developed properly. Such distortion is known as crossmodulation distortion. The common method of testing for crossmodulation distortion involves the recording of a high frequency test signal, amplitude modulated by a low frequency test signal on the optical sound track. For 35 mm motion picture sound tracks, 8000-9000 hertz and 400 hertz are commonly used for the high and low frequency test signals, respectively.
A segment of a film strip 2 having recorded thereon such a test sound track in dual bilateral format is shown in FIG. 1. One wavelength of the low frequency test signal is denoted by dimension A. The amplitude modulated high frequency test signal appears within each lobe 3 of the low frequency signal.
If there is no crossmodulation distortion, only the originally recorded signal with frequencies present only around the high frequency test signal will be present when the sound track is played back. For example, with a high frequency signal of 9,000 hertz and conventional amplitude modulation used to create the crossmodulation signal, the resultant frequencies will be at 8,600, 9,000 and 9,400 hertz. The presence of crossmodulation distortion results in the presence of not only the above three frequencies during playback, but also a spurious signal at 400 hertz. FIG. 2 depicts an enlarged portion of the high frequency test signal image. For zero crossmodulation distortion, the test signal appears as an amplitude modulated sine wave, shown in dashed lines 4. The portion of the sound track within this signal envelope is darkened, as indicated by the diagonal lining. The presence of crossmodulation distortion causes the image boundary to shift, for example to the boundary indicated by solid line 6. In this case the lined area between lines 4 and 6 would also be dark on the sound track, distorting the recorded signal and leading to lower quality sound reproduction.
If both the recording and printing of the crossmodulation test signals are done properly, the amplitude modulated signal will play back as recorded and the sound track will exhibit no distortion. If, however, either the recording or printing was imperfect, the playback will contain a residual crossmodulation signal having a frequency equal to that of the low frequency modulating signal. The level of the spurious residual signal provides a measure of the distortion level. Its phase relative to the phase of the low frequency modulating test signal indicates whether the print and/or negative has been overexposed or underexposed.
The crossmodulation distortion test that has been used in the industry involves the recording of several feet of test signal on a film, and then running the film through a conventional optical sound track playback device at a controlled speed to measure the crossmodulation distortion signal with a filter tuned to the frequency of the low frequency modulating signal. Because this technique is cumbersome and time consuming, it is performed only infrequently. Furthermore, it provides only an average indication of crossmodulation distortion, rather than a continuous indication of the distortion level at discrete locations along the track. This can result in certain crossmodulation distortion being missed, and also severely limits the usefulness of the testing technique as a diagnostic tool for determining the cause of the distortion. Further details on crossmodulation distortion testing may be obtained from a publication by Howard M. Tremaine, Audio Cyclopedia, sections 18.283-18.299, Howard W. Sams & Co., 1977 (2d edition).